Many hundreds of strains of Bacillus thuringiensis (B.t.) produce insecticidal toxins designated as delta endotoxins. They are synthesized by sporulating B.t. cells. When toxin is ingested by a susceptible insect, the cells of the gut epithelium are destroyed.
The reported activity spectrum of B.t. covers insect species within the orders Lepidoptera and Coleoptera, many of which are major pests in agriculture and forestry. The activity spectrum also includes the insect order Diptera, which includes mosquitoes and black flies. See Couch, T. L. (1980) "Mosquito Pathogenicity of Bacillus thuringiensis var. israelensis," Developments in Industrial Microbiology 22:61-76; Beegle, C. C. (1978) "Use of Entomogenous Bacteria in Agroecosystems," Developments in Industrial Microbiology 20:97-104. Dipteran insects are serious nuisances as well as being vectors of many serious human and animal diseases such as malaria, onchocerciasis, equine encephalitis, and dog heartworm.
Approximately 9.3 million acres of U.S. corn is infested with the corn rootworm species complex, which includes the northern corn rootworm, Diabrotica barberi, the southern corn rootworm, D. undecimpunctata howardi, and the western corn rootworm, D. virgifera virgifera. The soil-dwelling larvae of these Diabrotica species feed on corn root, causing lodging of the corn plant. This eventually results in yield reduction or death of the plant. By feeding on cornsilks, the adults reduce pollination and, therefore, the yield of corn per plant. In addition, adults and larvae of the southern corn rootworm, also known as the spotted cucumber beetle, attack cucurbit crops (cucumbers, melons, squash, etc.) and many vegetable and field crops in commercial production as well as in home gardens.
Control of the corn rootworm has been partially addressed by cultural methods, such as crop rotation and application of high nitrogen levels to stimulate the growth of adventitious root systems. However, chemical insecticides are relied upon heavily to guarantee the desired level of control. Insecticides are banded onto the soil or incorporated into the soil. The major problem associated with the use of these chemicals is the development of resistance among the treated insect populations.
U.S. Pat. No. 4,849,217 discloses Bacillus thuringiensis isolates active against the alfalfa weevil.
The alfalfa weevil, Hypera postica, and the closely related Egyptian alfalfa weevil, Hypera brunneipennis, are the most important insect pests of alfalfa grown in the United States, with 2.9 million acres infested in 1984. An annual sum of 20 million dollars is spent to control these pests. The Egyptian alfalfa weevil is the predominant species in the southwestern U.S., where it undergoes aestivation (i.e., hibernation) during the hot summer months. In all other respects, it is identical to the alfalfa weevil, which predominates throughout the rest of the U.S.
The larval stage is the most damaging in the weevil life cycle. By feeding at the alfalfa plant's growing tips, the larvae cause skeletonization of leaves, stunting, reduced plant growth, and, ultimately, reductions in yield. Severe infestations can ruin an entire cutting of hay. The adults, also foliar feeders, cause additional, but less significant, damage.
Chemical insecticides play a major role in effective alfalfa weevil control. However, there are several problems associated with their use including:
1. acute mammalian toxicity: several of the most effective insecticides used for weevil control are highly toxic to humans and other mammals, and are sold on a restricted basis in many states. Toxic residues are an additional problem for hay sold as feed for livestock.
2. honeybee toxicity: the honeybee is sensitive to some of the insecticides used for alfalfa weevil control. Because alfalfa is the major source of nectar for commercial honeybee colonies in the U.S., the use of insecticides with honeybee toxicity is incompatible with the needs of the honey producers.
3. toxicity to natural enemies: the insect parasites and predators which normally help control populations of minor alfalfa pests (aphids, spider mites, leafhoppers, caterpillars) are highly susceptible to all insecticides presently used for alfalfa weevil control. Reductions in the numbers of beneficial insects can result in increased populations of these once minor pests (secondary pests outbreaks), and in the consequent application of additional insecticides. Secondary pest outbreaks of aphids and mites often lead to serious yield reductions.
At present there is a need for more effective control agents, particularly efficacious agents that act selectively and do not cause the secondary outbreaks of mites and aphids that can be so devasting to alfalfa.